1. Field of the Invention
The present invention relates to a gradation converting circuit for image signal processing and, more specifically, it relates to a digital gradation converting circuit for image signal processing which reduces tone discontinuities, false contour lines, and so on.
2. Description of the Prior Art
FIG. 1 depicts the relation between density range of an original and the density range of a printed matter. Color films, which occupy most of color originals, have a density range of 2.4 to 3.5. However, the density range in printed reproductions of originals is about 1.8 to 2.0. Therefore, the density range of printed matter must be compressed with respect to the original thereof. In FIG. 1, the line A is a density curve of a case where the entire range is simply compressed. In this case, the printed matter is faint in general. Therefore, the highlight to intermediate density range including important objects should not be so much compressed as the shadow portion. The example of that case is shown by the curve B. The reproduction of an image wherein with the degree of compression is changed partially is referred to as tone correction, and a circuit for carrying out the tone correction is called a gradation converting circuit.
A gradation converting circuit (also referred to as a gradation circuit, density range correction circuit) as described above is indispensable to scanners, a common image input apparatuses or in an image processing apparatuses.
Generally, the gradation converting circuit provides an output data y=f (x) using a function f having a desired conversion characteristics in association with an input data x. The gradation converting circuit comprises both analog gradation converting circuits and digital gradation converting circuits. The input data x and the output y are data related to density (or reflectance, transmittance, brightness and so on).
An analog circuit is disadvantageous in that arbitrary functional characteristics cannot be obtained, and in that the circuit becomes unstable due to resistance fluctuations caused by temperature variations. A digital circuit can supply output data from a memory to produce an arbitrary function in response to input data applied to the address lines of the memory, producing a so-called lookup table (LUT). When a LUT is used, the speed of processing of the gradation conversion is determined by the access time of the memory, allowing the speed to be increased while maintaining stability, eliminating the problems of the analog circuit. The aforesaid memory may be an RAM (random access memory) or an ROM (read only memory). In case of an RAM, the functional characteristics that is the output produced by the RAM can be easily changed, e.g. by reprogramming the contents of the RAM.
However, in a digital gradation converting circuit which employs a lookup table is susceptible of producing quantization errors. This is not a problem in an analog circuit. More specifically, in the above described and similar gradation converting circuits, the non-linear characteristics of the gradation curve is formed by means of the lookup table, so that errors in quantization are generated in the output data.
In order to briefly explain the above described problem, a simple example will be described with reference to FIG. 2 in which each of the input, and output data are respectively constituted of 4 bits. In the field of photolithography and the like requiring high precision, the lookup table is usually formed with 8 bits. The larger the number of bits, the smoother becomes the gradation curve.
Referring to FIG. 2, the input/output converting characteristic of the basic data of the outputs to the inputs x is represented by a smooth curve A. However, the actual characteristic has a 1 bit uncertainty producing the stepwise varying output denoted by the line B, as a result of the digital nature of the circuit.
For example, for x=6, the curve A in FIG. 1 yield an output 9.75. However, in digital system, the actual value will be y=10, the basic data being rounded off. Obtaining such a stepwise line B is disadvantageous in tone reproduction applications.
In FIG. 2, when the input data x changes from 0 to 1, the output data y rapidly changes from 0 to 3. When x changes from 1 to 2, y changes from 3 to 5. Consequently, in the displayed image and undesired phenomenon called tone jump or false contour line is generated in which the gradation changes abruptly.
In contrast to the foregoing, when x changes from 8 to 9, y remains at 12. Similarly, y is not changed when x changes from 11 to 12. At these portions of the curve, delicate differences of gradation cannot be reproduced.
In order to solve the problem, the number of bits of the data x and y should be increased to minimize errors. However, if the number of bits is increased, the circuit becomes complicated and the cost thereof is increased. Therefore, increasing the bit number is impractical.
Therefore, in order to reduce the errors in quantization, a method has been proposed in which random numbers (noise) are mixed with the data to minimize the influence of the errors in quantization as shown in FIGS. 3A and 3B.
In FIG. 3A, a random number, outputted at random from a random number generating circuit 21, is added to the input data x by an adder 22. The added data designates an address in the lookup table LUT, and an output data y is outputted in correspondence with the designated address.
In FIG. 3B, an address for the lookup table LUT is designated by an input data x and a random number data outputted at random from the random number generating circuit 21 is added to the data outputted from the lookup table LUT by the adder 22, with the result of the addition outputted as the output data y.
In the above described methods, noise numbers are directly mixed with the signals. Therefore, as the random numbers of larger bits are added to reduce the tone jump or false contour line, excessive noise is introduced, and the image quanlity suffers.